1. Field of the Invention
The invention relates to an ultra-wideband antenna systems working both in sending or transmission mode and in reception mode without re-matching. It is designed for example for the frequencies band varying from 30 to 512 MHz, from the VHF (Very High Frequencies) up to the UHF (Ultra High Frequencies)
This band encompasses especially the classic bands: the usual VHF-FM or VHF frequency modulation band ranging from 30 to 88 MHz, the VHF-AM or VHF amplitude modulation band ranging from 100 to 160 MHz and the UHF-AM (UHF-amplitude modulation) band ranging from 225 to 400 MHz.
2. Description of the Prior Art
Modern transmission-reception (T/R) stations are capable of working in all the frequency bands varying from 30 to 512 MHz. However, the existing antenna systems that are associated with them, especially those designed to be installed in mobile telephones, provide optimum operation only in the active frequency bands, for example the VHF-FM (30–88 MHz) band or the VHF-AM (100–160 MHz) band or again the UHF-AM (225–400 MHz) band. Consequently, the exploitation of these transceivers necessitates the implementation of several antennas and a switching device to select the most suited antenna.
Ultra-wideband antenna systems enabling the discontinuity-free coverage of the entire 30 to 512 MHz band already exist for other systems using radio-frequencies, for example radiocommunications air-borne antenna systems, antenna systems for listening and scrambling in electronic warfare etc. However, these antennas have drawbacks which make them unsuited for use in a land-based moving object. Indeed, their efficiency is either too low for the radio range required for ground-ground links, as in the case of air-borne antennas, or their space requirement is incompatible with the dimensions of the vehicle.
There are numerous antenna structures covering a band of frequencies attaining the decade and having low space requirement. For example the patents U.S. Pat. Nos. 4,443,803, 4,466,003 or 4,958,164 describe such structures. However, the structures are all based on the contribution of resistive elements to the artificially widening of the antenna bandwidth. Consequently, a large part of the radio-frequency (RF) power is not radiated by the antenna but converted into heat within it. The efficiency of these types of antenna therefore proves to be very low. Another fault of the radiating structures described in the above-mentioned patents is their poor radiation pattern for the high frequencies.
The patents DE 3 826 777 or FR 2 758 012 propose structures known as multiband structures, that cover several frequency bands and do not have the above-stated defects. However, the frequency bands covered by these types of antenna must imperatively be discontinuous.